1. Field of the Invention
This invention relates to soft hydrogel contact lenses and more specifically, is directed to soft contact lens design.
2. Description of the Prior Art
As is known in the art, contact lenses are frequently made from polymethyl methacrylate. Such lenses are known as the "hard lenses". Many people cannot adapt to the presence of a hard lens in the eye and with others, the lens compromises the physiological processes required for corneal metabolism. More recently, new soft lens materials have been developed which avoid some of the problems associated with the hard lens. One class of such lens materials is described in U.S. Pat. Nos. 2,976,576 and 3,220,960 incorporated herein by reference. These materials are hydrogels of a sparingly cross-linked hydrophilic copolymer comprising a major amount of a monoester of an olefinic acid from the group of acrylic and methacrylic acids having a single olefinic double bond and a minor amount of a polymerizable diester of one of said acids, the diester having at least two olefinic double bonds. A preferred hydrogel disclosed in the aforesaid patent is a slightly cross-linked material comprising a predominant quantity of 2-hydroxyethyl methacrylate. The hydrogel, known as "hema", is used for contact lens fabrication because of its ability to absorb water of hydration, typically from about 35 to 65% by weight of the hydrogel. The water renders the lens flexible and soft which properties enable it to mold to the curvature of the eye. This is in contrast to the conventional hard lens which maintains a rigid configuration that does not always conform to the eye's curvature.
In U.S. patent application Ser. No. 685,996, filed May 13, 1976, assigned to the same assignee as the subject application and incorporated herein by reference, hydrogels are disclosed which are also suitable for soft lens fabrication. The hydrogels are formed from a hydrophilic monomer from the group of dihydroxyalkyl acrylates and methacrylates (collectively the "dihydroxy acrylate"), a substantially water insoluble monomer from the group of alkyl acrylates and methacrylates (collectively "the acrylate") and preferably, a minor amount of an epoxidized alkyl acrylate or methacrylate (collectively the "epoxidized acrylate"). The dihydroxyalkyl acrylate is preferably used in major amount, the alkyl acrylate in minor amount, and the epoxidized acrylate in an amount sufficient to impart the desired rigidity. The polymer is formed by a free radical, bulk polymerization process in the substantial absence of solvent in order to obtain a polymer suitable for fabrication of a contact lens.
Numerous other soft lens materials are known and include cross-linked poly(hydroxyethyl methacrylate) and a cross-linked copolymer of hydroxyethyl methacrylate grafted onto a poly(vinyl pyrrolidone) backbone. Still other soft lens materials and processes for their formation are reported in the following U.S. Pat. Nos. 3,532,679; 3,639,524; 3,647,736; 3,721,657; 3,758,448; 3,767,731; 3,772,235; 3,803,093; 3,816,571; 3,822,196; 3,839,304 and 3,841,985, all incorporated herein by reference.
It is known in the art that the conventional hard contact lenses and many of the contemporary soft hydrogel lenses may only be worn for a short duration of time, typically for periods of time up to twelve hours. It is further known that extended wear, for periods in excess of twelve hours, particularly during closed-eye periods (during sleeping hours) may cause long term injury to the eye.
One cause of injury resulting from extended wear of the above lenses is the lens resting on the bulbar conjunctivia for an extended time. Due to the weight of the lens, the peripheral areas of the lens compress the vascular system thereby compressing the limbal capillaries causing physiological complications. One approach at solving this problem has been the use of wider band and flatter posterior curvatures which result in looser fitting lens. However, this approach has not been notably successful because the loose fit results in lens movement when blinking occurs.
Another cause of injury arising from extended wear of such lenses is corneal-scleral wetting deficiences resulting in dry areas of the corneal-scleral tissue. This problem may be caused by alterations in the normal blinking mechanism that either reduces the frequency of blinks or alters the ability of the lids to make a complete closure upon blinking. Additionally, many lens designs create breaks in the normal lid cornea contact necessary for the required evenly distributed wiping and spreading of the tears over all of the corneal-scleral tissue. This problem is usually encountered in the areas of the tissue immediately peripheral to the edge of the lens.
Perhaps the most serious cause of injury arising from extended wear of the aforesaid lenses is oxygen deprivation due to the lens covering a significant portion of the corneal surface thus acting as a barrier to contact of the cornea with an oxygenated tear layer. This results in oxygen deprivation at the cornea and interferes with the metabolic and physiological requirements of the cornea.
It is known that the cornea requires a supply of oxygen at its surface and relies upon oxygen diffusion from a tear layer over its surface for almost all of its required supply. During open-eye periods, the tear layer is oxygenated by atmospheric oxygen. During closed-eye periods, the tear layer is oxygenated by the capillaries of the eyelid rather than oxygen from the atmosphere as when the eye is open. The partial pressure of oxygen supplied from the capillaries is less than about 1/3 that supplied by the atmosphere. In the absence of a lens acting as a barrier, the oxygen supply to the cornea, both during open-eye and closed-eye periods, is sufficient. In this respect, Mandell and Polse, in "Critical Oxygen Tension at the Corneal Surface", Archives of Opthamology, 84, 505 (1970), have determined that the minimum partial pressure of oxygen needed at the surface of the cornea to maintain normal corneal physiology is 15 mm Hg or a supply rate of about 3.5 .mu.l/cm.sup.2 -hr. Others have stated that this minimum rate is 6.0 .mu.l/cm.sup.2 -hr.
From the aforesaid, it is clear that a contact lens capable of continuous wear should provide at least 3.5 .mu.l/cm.sup.2 -hr, preferably 6 .mu.l/cm.sup.2 -hr of oxygen to the corneal surface to avoid the physiological complication arising from oxygen deprivation. Hard contact lenses, such as those of methyl methacrylate, are not permeable to oxygen, but through known lens design, permit some circulation of air to the corneal surface. Such design features may comprise a tear pump mechanism whereby oxygenated tears are pumped beneath the lens with each blink of the eyelid and/or central or paracentral apertures whereby there is actual atmospheric contact with the cornea. Contemporary hydrogel lenses, though permeable to oxygen through the interstitial spaces of the hydrogel material, are not sufficiently permeable to fully oxygenate the cornea in the cross-sections in which they are fabricated. Hence, oxygen deprivation is also encountered with these lenses.
The circulation of some oxygen using the aforesaid lenses permits daily wear of the same with minimal non-reversible damage to corneal physiology. However, during closed-eye periods, when the oxygen supply is reduced to less than 1/3 the level of opened-eye condition, known lens design does not permit sufficient transfer of oxygen to the corneal surface to permit wear without oxygen deprivation.
The above problems have caused the art to seek alternative means for supplying required oxygen to the corneal surface. One such attempt is described in U.S. Pat. No. 3,551,035, incorporated herein by reference. In this patent, a rigid (hard) lens material is used which material, unlike the methyl methacrylate hard lens, is permeable to oxygen. However, this lens suffers other disadvantages of hard lenses and consequently, in U.S. Pat. No. 3,619,044, also incorporated herein by reference, the patentee attempts to overcome the disadvantages of the hard lens by grafting soft lens material onto the peripheral edge of the hard lens. This procedure is obviously expensive and the final product does not overcome other of the known problems encountered in the extended wear of a contact lens.
Another approach to the problem has been the use of permeable lenses fabricated from silicone resins and copolymers of silicone resins with methyl methacrylate. This approach has not met with success insofar as extended wear is concerned because of the weight of the lens and the damage resulting to the bulbar conjunctivia as a consequence of the weight as described above.
Additional research has been performed both on lens materials and lens design in an effort to achieve a lens suitable for continuous wear. However, it is believed that no lens now available avoids the disadvantages inherent in continuous wear as described above.